One of the main ways that we breathe is with our chest. The muscles between the ribs cause the ribcage to expand and lift up, drawing air into the lungs. This causes the abdominal muscles to become long as they are pulled by the upward motion of the ribs. The abdominal muscles must be relaxed for the ribs to lift fully.

As the abdominal muscles are pulled and the abdominal cavity is drawn upward, their circumference is reduced, essentially bringing the abdomen in. This appearance is misleading, since from the outside it looks as if the abdominal muscles are contracting to "pull the belly in." They are not.

If the abdominal muscles contract, they will pull down on the ribcage, preventing it from lifting and the breath from coming in. This means that the more the abdominal muscles are engaged, the less effective our breathing will be. It is actually the rib muscles (external intercostals) that lift up and pull the abdomen into its tight-looking appearance.

Whenever you breathe into your chest, focus on expanding the ribcage and relax the abdominal muscles as much as possible. You will breathe deeper with far less effort.